JP2019533083A - Cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability, hot-formed member, and manufacturing method thereof - Google Patents

Cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability, hot-formed member, and manufacturing method thereof Download PDF

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JP2019533083A
JP2019533083A JP2019515875A JP2019515875A JP2019533083A JP 2019533083 A JP2019533083 A JP 2019533083A JP 2019515875 A JP2019515875 A JP 2019515875A JP 2019515875 A JP2019515875 A JP 2019515875A JP 2019533083 A JP2019533083 A JP 2019533083A
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hot
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rolled steel
steel sheet
corrosion resistance
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JP6872009B2 (en
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ジン−グン オ、
ジン−グン オ、
ヨル−レ チョ、
ヨル−レ チョ、
ア−ラ チョ、
ア−ラ チョ、
ジン−ホ チャ、
ジン−ホ チャ、
シ−ミョン ホ、
シ−ミョン ホ、
ジョン−ウォン ソ、
ジョン−ウォン ソ、
ソン−ウ キム、
ソン−ウ キム、
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Posco Holdings Inc
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Posco Co Ltd
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
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    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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Abstract

本発明の一側面は、重量％で、Ｃ：０．１〜０．４％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．０１〜４．０％、Ａｌ：０．００１〜０．４％、Ｐ：０．００１〜０．０５％、Ｓ：０．０００１〜０．０２％、Ｃｒ：０．５％以上３．０％未満、Ｎ：０．００１〜０．０２％、残りＦｅ及びその他の不可避不純物を含み、下記式（１）を満たし、表面に連続的または不連続的に１ｎｍ〜１００ｎｍの厚さのＳｉ非晶質酸化層が形成されている、耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板に関するものである。式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２（上記式（１）において各元素記号は、各元素の含量を重量％で測定した値である。）One aspect of the present invention is, by weight, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001- 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.5% or more and less than 3.0%, N: 0.001 to 0.02% Corrosion resistance and spots, including the remaining Fe and other inevitable impurities, satisfying the following formula (1), and having a Si amorphous oxide layer having a thickness of 1 nm to 100 nm formed continuously or discontinuously on the surface The present invention relates to a cold-rolled steel sheet for hot forming excellent in weldability. Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2 (In the above formula (1), each element symbol is a value obtained by measuring the content of each element in weight%.)

Description

本発明は、耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板、熱間成形部材及びその製造方法に関する。   The present invention relates to a cold-rolled steel sheet for hot forming, a hot-formed member, and a method for producing the same, which are excellent in corrosion resistance and spot weldability.

熱間成形により製造された部材は、最近、自動車の軽量化による燃費向上及び超高強度化による衝突性向上のために、自動車の構造部材に多く適用されており、これに対する研究が多く行われている。   Recently, members manufactured by hot forming have been widely applied to structural members of automobiles in order to improve fuel efficiency by reducing the weight of automobiles and improve collision performance by increasing strength. ing.

代表的な技術としては、特許文献１に提示された発明を挙げることができる。   As a representative technique, the invention presented in Patent Document 1 can be cited.

特許文献１では、Ａｌ−Ｓｉめっき鋼板を８５０℃以上の温度で加熱した後、プレスで熱間成形及び急冷を行って部材の組織をマルテンサイトに形成することにより、引張強度が１６００ＭＰａを超える超高強度を確保できることを提示している。また、熱処理中に母材からめっき層へのＦｅの拡散によって形成された合金化層及び拡散層により、ショットブラストがなくても耐食性及びスポット溶接性を確保することができる。   In Patent Document 1, after heating an Al—Si plated steel sheet at a temperature of 850 ° C. or higher, hot forming and rapid cooling are performed with a press to form a structure of the member into martensite, whereby the tensile strength exceeds 1600 MPa. It shows that high strength can be secured. Further, the alloying layer and the diffusion layer formed by diffusion of Fe from the base material to the plating layer during the heat treatment can ensure corrosion resistance and spot weldability even without shot blasting.

しかし、Ａｌ−Ｓｉめっき層を形成しなければならないため、別のめっき工程が必要となり、経済性及び生産性が低下するという問題がある。   However, since an Al—Si plating layer must be formed, another plating process is required, and there is a problem that economic efficiency and productivity are lowered.

一方、非めっき材の場合、熱処理時に生成される酸化層によりスポット溶接性を確保することができない問題がある。そのため、酸化層を除去するためのショットブラスト工程が必要となるだけではなく、耐食性を確保し難いという問題がある。   On the other hand, in the case of a non-plated material, there is a problem that spot weldability cannot be ensured by an oxide layer generated during heat treatment. Therefore, not only a shot blasting process for removing the oxide layer is required, but also there is a problem that it is difficult to ensure corrosion resistance.

したがって、めっき工程及びショットブラスト工程がなくても、優れた耐食性及びスポット溶接性を確保することができる熱間成形用冷延鋼板、熱間成形部材及びその製造方法に関する開発が求められているのが実情である。   Therefore, there is a need for development related to a hot-formed cold-rolled steel sheet, a hot-formed member, and a manufacturing method thereof that can ensure excellent corrosion resistance and spot weldability without a plating process and a shot blasting process. Is the actual situation.

米国特許第６２９６８０５号明細書US Pat. No. 6,296,805

本発明の一側面は、耐衝突性が求められる自動車構造部材または補強材などに好適に適用することができ、めっき工程及びショットブラスト工程がなくても優れた耐食性とスポット溶接性を確保することができる熱間成形用冷延鋼板、熱間成形部材及びその製造方法を提供することを目的とする。   One aspect of the present invention can be suitably applied to automobile structural members or reinforcing materials that require impact resistance, and ensures excellent corrosion resistance and spot weldability without a plating step and a shot blasting step. An object is to provide a cold-rolled steel sheet for hot forming, a hot-formed member, and a method for producing the same.

一方、本発明の課題は、上述の内容に限定されない。本発明の課題は、本明細書の内容全般から理解できるものであり、本発明が属する技術分野における通常の知識を有する者であれば、本発明の付加的な課題を理解するのに何ら困難はない。   On the other hand, the subject of this invention is not limited to the above-mentioned content. The problems of the present invention can be understood from the entire contents of the present specification, and it is difficult for those who have ordinary knowledge in the technical field to which the present invention belongs to understand the additional problems of the present invention. There is no.

本発明の一側面は、重量％で、Ｃ：０．１〜０．４％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．０１〜４．０％、Ａｌ：０．００１〜０．４％、Ｐ：０．００１〜０．０５％、Ｓ：０．０００１〜０．０２％、Ｃｒ：０．５％以上３．０％未満、Ｎ：０．００１〜０．０２％、残りＦｅ及びその他の不可避不純物を含み、下記式（１）を満たし、
表面に連続的または不連続的に１〜１００ｎｍの厚さのＳｉ非晶質酸化層が形成されている、耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板に関するものである。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
（上記式（１）において各元素記号は、各元素の含量を重量％で測定した値である。） One aspect of the present invention is, by weight, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001- 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.5% or more and less than 3.0%, N: 0.001 to 0.02% , Including the remaining Fe and other inevitable impurities, satisfying the following formula (1),
The present invention relates to a cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability, in which a Si amorphous oxide layer having a thickness of 1 to 100 nm is formed on the surface continuously or discontinuously.
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
(In the above formula (1), each element symbol is a value obtained by measuring the content of each element in% by weight.)

また、本発明の他の一側面は、重量％で、Ｃ：０．１〜０．４％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．０１〜４．０％、Ａｌ：０．００１〜０．４％、Ｐ：０．００１〜０．０５％、Ｓ：０．０００１〜０．０２％、Ｃｒ：０．５％以上３．０％未満、Ｎ：０．００１〜０．０２％、残りＦｅ及びその他の不可避不純物を含み、下記式（１）を満たすスラブを１０００〜１３００℃の温度で加熱する段階と、
前記加熱されたスラブをＡｒ３〜１０００℃の仕上げ圧延温度で熱間圧延して熱延鋼板を得る段階と、
上記熱延鋼板をＭｓ超７５０℃以下の温度範囲で巻き取る段階と、
上記巻き取られた熱延鋼板を冷間圧延して冷延鋼板を得る段階と、
上記冷延鋼板を、下記式（２）及び式（３）を満たすように連続焼鈍する段階と、を含む、耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板の製造方法に関するものである。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
式（２）：１≦ｅｘｐ［０．０７＊ＤＰ（Ｉ）＋（０．６＊Ｃｒ＋３＊Ｓｉ）］≦１００
式（３）：５０＊ｅｘｐ［０．０５＊ＤＰ（Ｉ）−（１．２＊Ｃｒ＋６＊Ｓｉ）］≦２．５
（上記式（１）から式（３）において各元素記号は、各元素の含量を重量％で測定した値であり、式（２）及び式（３）においてＤＰ（Ｉ）は、上記連続焼鈍時の露点温度（℃）である。） Another aspect of the present invention is weight percent, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% or more and less than 3.0%, N: 0.001- Heating a slab containing 0.02%, the remaining Fe and other inevitable impurities and satisfying the following formula (1) at a temperature of 1000 to 1300 ° C .;
Hot-rolling the heated slab at a finishing rolling temperature of Ar 3 to 1000 ° C. to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet in a temperature range of more than Ms and 750 ° C. or less;
Cold rolling the rolled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
The present invention relates to a method for producing a hot-rolled cold-rolled steel sheet excellent in corrosion resistance and spot weldability, comprising the step of continuously annealing the cold-rolled steel sheet so as to satisfy the following formulas (2) and (3): is there.
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
Formula (2): 1 ≦ exp [0.07 * DP (I) + (0.6 * Cr + 3 * Si)] ≦ 100
Formula (3): 50 * exp [0.05 * DP (I) − (1.2 * Cr + 6 * Si)] ≦ 2.5
(In the above formulas (1) to (3), each element symbol is a value obtained by measuring the content of each element in% by weight. In formulas (2) and (3), DP (I) is the above-mentioned continuous annealing. (The dew point temperature of the hour (℃).)

また、本発明のさらに他の一側面は、本発明の冷延鋼板を用いて製造された熱間成形部材及びその製造方法に関するものである。   Still another aspect of the present invention relates to a hot-formed member manufactured using the cold-rolled steel sheet of the present invention and a manufacturing method thereof.

なお、上述の課題の解決手段は、本発明の特徴をすべて列挙したものではない。本発明の様々な特徴とそれによる利点と効果は、以下の具体的な実施形態を参照することにより、より詳細に理解することができる。   Note that the means for solving the problems described above do not enumerate all the features of the present invention. Various features of the present invention and its advantages and advantages can be better understood with reference to the following specific embodiments.

本発明によると、めっき工程及び熱間成形部材の製造時に表面に形成される酸化物を除去するためのショットブラスト工程がなくても、優れた耐食性及びスポット溶接性を確保できる熱間成形用冷延鋼板、熱間成形部材及びその製造方法を提供することができるという効果がある。また、１０００ＭＰａ以上の引張強度を確保することができる。   According to the present invention, cold for hot forming that can ensure excellent corrosion resistance and spot weldability without a plating step and a shot blasting step for removing oxides formed on the surface during the production of hot formed members. There exists an effect that a rolled steel plate, a hot forming member, and its manufacturing method can be provided. Moreover, the tensile strength of 1000 MPa or more can be ensured.

式（１）の値による表面等級の変化を示したグラフである。It is the graph which showed the change of the surface grade by the value of Formula (1). （ａ）は、式（２）の値による冷延鋼板のＳｉ非晶質酸化層の厚さの変化を示したグラフであり、（ｂ）は、式（３）の値による冷延鋼板の（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さの変化を示したグラフである。(A) is the graph which showed the change of the thickness of the Si amorphous oxide layer of the cold-rolled steel sheet by the value of Formula (2), (b) of the cold-rolled steel sheet by the value of Formula (3) It is the graph which showed the change of the thickness of a (Fe, Mn, Cr) oxide layer. （ａ）は、式（４）の値による熱間成形部材のＳｉ非晶質酸化層の厚さの変化を示したグラフであり、（ｂ）は、式（５）の値による熱間成形部材の（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さの変化を示したグラフである。(A) is the graph which showed the change of the thickness of Si amorphous oxide layer of the hot forming member by the value of Formula (4), (b) is the hot forming by the value of Formula (5) It is the graph which showed the change of the thickness of the (Fe, Mn, Cr) oxide layer of a member. 冷延鋼板の発明例Ａ２の表層構造を示す。The surface layer structure of invention example A2 of a cold-rolled steel sheet is shown. 冷延鋼板の発明例Ａ２の表層成分分布を示す。The surface layer component distribution of Invention Example A2 of the cold-rolled steel sheet is shown. 熱間成形部材の発明例Ａ２の表層構造を示す。The surface layer structure of invention example A2 of a hot forming member is shown. 熱間成形部材の発明例Ａ２の表層成分分布を示す。The surface layer component distribution of Invention Example A2 of a hot-formed member is shown.

以下、本発明の好ましい実施形態を説明する。しかし、本発明の実施形態は、様々な他の形態に変形されることができ、本発明の範囲が以下で説明する実施形態に限定されるものではない。また、本発明の実施形態は、当該技術分野における平均的な知識を有する者に本発明をさらに完全に説明するために提供されるものである。   Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. Also, the embodiments of the present invention are provided to more fully explain the present invention to those having average knowledge in the art.

熱間成形用非めっき冷延鋼板の場合、熱処理時に生成される酸化層によりスポット溶接性を確保することができない。そのため、酸化層を除去するためのショットブラスト工程が必要となるだけではなく、耐食性を確保し難いという問題がある。そこで、本発明者らは上記問題を認知し、それを解決するために鋭意研究した。   In the case of a non-plated cold-rolled steel sheet for hot forming, spot weldability cannot be ensured by an oxide layer generated during heat treatment. Therefore, not only a shot blasting process for removing the oxide layer is required, but also there is a problem that it is difficult to ensure corrosion resistance. Therefore, the present inventors have recognized the above problem and conducted intensive research to solve it.

その結果、合金組成及び製造条件、特にＣｒ含量、Ｓｉ含量、露点温度の相関関係を精密に制御して目標とする厚さでＳｉ非晶質酸化層（Ｓｉ系非晶質酸化層）を形成させることにより、めっき工程及びショットブラスト工程がなくても、優れた耐食性及びスポット溶接性を確保できることを確認し、本発明を完成するに至った。   As a result, Si amorphous oxide layer (Si-based amorphous oxide layer) is formed with the target thickness by precisely controlling the correlation between alloy composition and manufacturing conditions, especially Cr content, Si content, and dew point temperature. As a result, it was confirmed that excellent corrosion resistance and spot weldability could be secured without the plating process and shot blasting process, and the present invention was completed.

以下、本発明の一側面による耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板について詳細に説明する。   Hereinafter, the cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability according to one aspect of the present invention will be described in detail.

本発明の一側面による耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板は、重量％で、Ｃ：０．１〜０．４％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．０１〜４．０％、Ａｌ：０．００１〜０．４％、Ｐ：０．００１〜０．０５％、Ｓ：０．０００１〜０．０２％、Ｃｒ：０．５％以上３．０％未満、Ｎ：０．００１〜０．０２％、残りＦｅ及びその他の不可避不純物を含み、下記式（１）を満たし、表面に連続的または不連続的に１ｎｍ〜１００ｎｍの厚さのＳｉ非晶質酸化層が形成されている。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
（上記式（１）において各元素記号は、各元素の含量を重量％で測定した値である。） The cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability according to one aspect of the present invention is C: 0.1-0.4%, Si: 0.5-2.0%, Mn : 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% or more Less than 3.0%, N: 0.001 to 0.02%, including remaining Fe and other inevitable impurities, satisfying the following formula (1), and having a thickness of 1 nm to 100 nm continuously or discontinuously on the surface A Si amorphous oxide layer is formed.
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
(In the above formula (1), each element symbol is a value obtained by measuring the content of each element in% by weight.)

まず、本発明の一側面による耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板の合金組成について詳細に説明する。以下、各元素の含量の単位は重量％である。   First, the alloy composition of the cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability according to one aspect of the present invention will be described in detail. Hereinafter, the unit of the content of each element is% by weight.

Ｃ：０．１〜０．４％
Ｃは、熱処理部材の強度を上昇させるための必須元素であって、適正に添加される必要がある。 C: 0.1 to 0.4%
C is an essential element for increasing the strength of the heat treatment member, and needs to be added appropriately.

Ｃ含量が０．１％未満の場合には、十分な強度を確保し難いため、０．１％以上添加されることが好ましい。一方、その含量が０．４％を超える場合には、熱延材を冷間圧延する際に熱延材の強度が高すぎて冷間圧延性に非常に劣るようになり、且つ、スポット溶接性が大きく低下する。したがって、Ｃ含量は０．４％以下であることが好ましい。より好ましい上限は０．３５％であり、さらに好ましい上限は０．３％である。   When the C content is less than 0.1%, it is difficult to ensure sufficient strength, so 0.1% or more is preferably added. On the other hand, if the content exceeds 0.4%, the strength of the hot-rolled material is too high when cold-rolling the hot-rolled material, and the cold-rollability becomes very poor, and spot welding is performed. The performance is greatly reduced. Therefore, the C content is preferably 0.4% or less. A more preferable upper limit is 0.35%, and a further preferable upper limit is 0.3%.

Ｓｉ：０．５〜２．０％
Ｓｉは、冷延鋼板を連続焼鈍ラインで焼鈍する際に冷延鋼板の表面に濃化してＳｉ非晶質酸化層を形成する重要な役割を果たすだけではなく、熱間成形工程で（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の形成を抑制して部材のスポット溶接性を確保する役割を果たす。 Si: 0.5 to 2.0%
Si not only plays an important role in forming a Si amorphous oxide layer by concentrating on the surface of the cold-rolled steel sheet when the cold-rolled steel sheet is annealed in a continuous annealing line (Fe, Mn, Cr) Plays the role of suppressing the formation of the oxide layer and ensuring the spot weldability of the member.

Ｓｉ含量が０．５％未満の場合には、上述の効果が不十分となるため、その下限は０．５％であることが好ましい。より好ましい下限は０．８％である。一方、その含量が２．０％を超える場合には、厚すぎるＳｉ非晶質酸化層を形成してスポット溶接性がむしろ低下するという問題がある。   When the Si content is less than 0.5%, the above-described effects become insufficient, so the lower limit is preferably 0.5%. A more preferred lower limit is 0.8%. On the other hand, when the content exceeds 2.0%, there is a problem that the Si amorphous oxide layer is formed too thick and the spot weldability is rather lowered.

Ｃｒ：０．５％以上３．０％未満
Ｃｒは、鋼板の硬化能を向上させるだけではなく、Ｓｉと適切に反応することで表層のＳｉ系非晶質酸化物層の安定的な形成を助ける役割を果たすことができる。 Cr: 0.5% or more and less than 3.0% Cr not only improves the hardenability of the steel sheet, but also forms a stable Si-based amorphous oxide layer by reacting appropriately with Si. Can play a role to help.

Ｃｒ含量が０．５％未満の場合には、上述の効果が不十分となる。一方、Ｃｒ含量が３．０％以上である場合には、その効果が飽和し、製造コストが上昇するという問題がある。   When the Cr content is less than 0.5%, the above effect is insufficient. On the other hand, when the Cr content is 3.0% or more, there is a problem that the effect is saturated and the manufacturing cost increases.

上記ＣｒとＳｉは、各元素の含量を満たすだけではなく、式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２を満たす必要がある。図１から確認できるように、式（１）の値が１．４未満の場合には熱間成形後、表面に均一な表面等級を確保し難く、式（１）の値が３．２を超える場合には、その効果が飽和するだけでなく、むしろ製造コストが上昇し、且つスポット溶接性に劣るようになるという問題がある。より好ましい式（１）の値の上限は３．０であり、さらに好ましい上限は２．５である。   The Cr and Si need not only satisfy the content of each element but also satisfy the formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2. As can be seen from FIG. 1, when the value of the formula (1) is less than 1.4, it is difficult to secure a uniform surface grade on the surface after hot forming, and the value of the formula (1) is 3.2. In the case of exceeding, there is a problem that not only the effect is saturated but also the manufacturing cost is increased and the spot weldability is deteriorated. A more preferable upper limit of the value of the formula (1) is 3.0, and a more preferable upper limit is 2.5.

Ｍｎ：０．０１〜４．０％
Ｍｎは、固溶強化効果を確保することができるだけではなく、熱間成形部材においてマルテンサイトを確保するための臨界冷却速度を下げるために添加される必要がある。 Mn: 0.01 to 4.0%
Mn can not only ensure the solid solution strengthening effect, but also needs to be added to lower the critical cooling rate for securing martensite in the hot-formed member.

Ｍｎ含量が０．０１％未満の場合には、上述の効果が不十分となる。一方、Ｍｎ含量が４．０％を超える場合には、熱間成形工程前に鋼板の強度が上昇しすぎるため、ブランキング作業が困難になるだけではなく、合金鉄の過剰な添加により原価が上昇し、且つスポット溶接性を劣化させるという欠点がある。より好ましい上限は３．０％であり、さらに好ましい上限は２．０％である。   When the Mn content is less than 0.01%, the above effects are insufficient. On the other hand, if the Mn content exceeds 4.0%, the strength of the steel sheet is increased too much before the hot forming process, so that not only the blanking operation becomes difficult, but the cost is increased due to excessive addition of alloy iron. There is a drawback that it increases and deteriorates the spot weldability. A more preferred upper limit is 3.0%, and a more preferred upper limit is 2.0%.

Ａｌ：０．００１〜０．４％
Ａｌは、Ｓｉと共に製鋼において脱酸作用を行って鋼の清浄度を向上させることができる。 Al: 0.001 to 0.4%
Al, together with Si, can be deoxidized in steel making to improve the cleanliness of the steel.

Ａｌ含量が０．００１％未満の場合には、上述の効果が不十分となり、その含量が０．４％を超える場合には、Ａｃ３温度が上昇しすぎて加熱温度を高めなければならないという問題がある。より好ましい上限は０．２％であり、さらに好ましい上限は０．１％である。   When the Al content is less than 0.001%, the above effect is insufficient, and when the Al content exceeds 0.4%, the Ac3 temperature rises too much and the heating temperature must be increased. There is. A more preferable upper limit is 0.2%, and a further preferable upper limit is 0.1%.

Ｐ：０．００１〜０．０５％
Ｐは不純物であり、その含量を０．００１％未満に制御するためには、多くの製造コストがかかり、その含量が０．０５％を超える場合には、熱間成形部材の溶接性が大きく低下することがある。より好ましい上限は０．０３％である。 P: 0.001 to 0.05%
P is an impurity, and it takes a lot of manufacturing costs to control its content to less than 0.001%. When the content exceeds 0.05%, the weldability of the hot-formed member is large. May decrease. A more preferred upper limit is 0.03%.

Ｓ：０．０００１〜０．０２％
Ｓは不純物であり、その含量を０．０００１％未満に制御するためには、多くの製造コストがかかり、その含量が０．０２％を超える場合には、部材の延性、衝撃特性、及び溶接性を阻害する。より好ましい上限は０．０１％である。 S: 0.0001 to 0.02%
S is an impurity, and it takes a lot of manufacturing costs to control its content to less than 0.0001%, and when its content exceeds 0.02%, the ductility, impact properties, and welding of the member Inhibits sex. A more preferred upper limit is 0.01%.

Ｎ：０．００１〜０．０２％
Ｎは不純物であり、その含量を０．００１％未満に制御するためには、多くの製造コストがかかり、その含量が０．０２％を超える場合には、スラブの連鋳時にクラックの発生に敏感になるだけではなく、衝撃特性が悪くなることがある。より好ましい上限は０．０１％である。 N: 0.001 to 0.02%
N is an impurity, and it takes a lot of manufacturing costs to control its content to less than 0.001%. If its content exceeds 0.02%, cracks may occur during continuous casting of slabs. Not only will it become sensitive, but impact characteristics may deteriorate. A more preferred upper limit is 0.01%.

本発明の残り成分は鉄（Ｆｅ）である。但し、通常の製造過程では、原料または周囲の環境から意図しない不純物が不可避に混入することがあるため、それを排除することはできない。これら不純物は、通常の製造過程における技術者であれば、誰でも分かるものであるため、そのすべての内容を具体的に本明細書に記載しない。   The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed from the raw material or the surrounding environment, and therefore cannot be excluded. Since these impurities can be understood by any engineer in the normal manufacturing process, the entire contents thereof are not specifically described in this specification.

このとき、下記ａ）及びｂ）の中から選択された１以上をさらに含むことができる。   At this time, it may further include one or more selected from the following a) and b).

ａ）Ｔｉ、Ｎｂ、Ｚｒ及びＶから選択された１種以上：０．００１〜０．４％
Ｔｉ、Ｎｂ、Ｚｒ及びＶは、微細析出物の形成による熱処理部材の強度の向上と、結晶粒の微細化による残留オーステナイトの安定化及び衝撃靭性の向上において効果がある。その含量（２種以上が追加された場合には、それらの合計を意味する）が０．００１％以下では、上述の効果が不十分となり、その含量が０．４％を超えると、その効果が飽和するだけでなく、合金鉄の過剰な添加によるコストの上昇を招くことがある。 a) One or more selected from Ti, Nb, Zr and V: 0.001 to 0.4%
Ti, Nb, Zr, and V are effective in improving the strength of the heat-treated member by forming fine precipitates, stabilizing the retained austenite by reducing the crystal grains, and improving the impact toughness. If the content (meaning the sum of two or more added) is 0.001% or less, the above effect is insufficient, and if the content exceeds 0.4%, the effect Not only becomes saturated but also increases in cost due to excessive addition of iron alloy.

ｂ）Ｂ：０．０００１〜０．０１％
Ｂは、少量の添加でも硬化能を向上させることができるだけではなく、旧オーステナイト結晶粒界に偏析し、Ｐ及び／またはＳの粒界偏析による熱間成形部材の脆性を抑制することができる元素である。 b) B: 0.0001 to 0.01%
B is an element that not only can improve the curability even when added in a small amount, but also segregates at the prior austenite crystal grain boundaries and suppresses brittleness of the hot-formed member due to P and / or S grain boundary segregation. It is.

Ｂ含量が０．０００１％未満の場合には、上述の効果が不十分となり、０．０１％を超える場合には、その効果が飽和するだけではなく、熱間圧延時に熱間脆性を引き起こすことがある。より好ましい上限は０．００５％である。   When the B content is less than 0.0001%, the above effect is insufficient, and when it exceeds 0.01%, the effect is not only saturated, but also causes hot brittleness during hot rolling. There is. A more preferred upper limit is 0.005%.

また、下記ｃ）からｅ）の中から選択された１以上をさらに含むことができる。   One or more selected from the following c) to e) may be further included.

ｃ）Ｍｏ及びＷから選択された１種以上：０．００１〜１．０％
Ｍｏ及びＷは、硬化能の向上と、析出強化の効果による強度の向上、及び結晶粒の微細化のために添加することができる。その含量（Ｍｏ及びＷが両方とも添加された場合には、その合計を意味する）が０．００１％未満の場合には、上述の効果が不十分となり、１．０％を超える場合には、その効果が飽和するだけでなく、コストが上昇するという問題がある。 c) One or more selected from Mo and W: 0.001 to 1.0%
Mo and W can be added for improving the hardening ability, improving the strength due to the effect of precipitation strengthening, and refining the crystal grains. When the content (when both Mo and W are added, the sum thereof) is less than 0.001%, the above effect is insufficient, and when it exceeds 1.0% The effect is not only saturated, but also the cost increases.

ｄ）ＣｕとＮｉ含量の合計：０．００５〜２．０％
Ｃｕは、微細析出物を形成させて強度を向上させる元素として添加されることができる。また、Ｎｉは、Ｃｕが単独で添加される場合に熱間脆性を引き起こすことがあるため、必要に応じて添加される。しかし、これら成分の合計が０．００５％未満では、上述の効果が不十分となり、２．０％を超えると、コストの過剰な上昇を招くことがある。 d) Total Cu and Ni content: 0.005 to 2.0%
Cu can be added as an element that improves the strength by forming fine precipitates. Ni is added as necessary because Cu may cause hot brittleness when Cu is added alone. However, if the total of these components is less than 0.005%, the above-described effects are insufficient, and if it exceeds 2.0%, the cost may be excessively increased.

ｅ）Ｓｂ及びＳｎから選択された１種以上：０．００１〜１．０％
上記Ｓｂ及びＳｎは、Ｓｉが添加された鋼材の熱延材表層の結晶粒界に生成され得る酸化物の生成を抑制する効果を有し、冷延材の焼鈍時に表層の結晶粒界の脱落によるデント（ｄｅｎｔ）欠陥を抑制することができる。かかる効果を得るためには、０．００１％以上添加することが好ましい。 e) one or more selected from Sb and Sn: 0.001 to 1.0%
Sb and Sn have the effect of suppressing the formation of oxides that can be generated at the grain boundaries of the hot-rolled steel surface layer of the steel material to which Si is added, and the crystal grain boundaries on the surface layer fall off during the annealing of the cold-rolled material. It is possible to suppress a dent defect caused by. In order to obtain such an effect, 0.001% or more is preferably added.

一方、その含量（Ｓｂ及びＳｎが両方とも添加された場合には、その合計を意味する）が１．０％を超えると、コストが上昇しすぎるだけでなく、スラブの粒界に固溶されて熱間圧延時にコイルエッジのクラックを引き起こすことがある。   On the other hand, if its content (meaning the sum of both when Sb and Sn are added) exceeds 1.0%, not only will the cost increase, but it will also be dissolved in the grain boundary of the slab. This may cause cracks at the coil edge during hot rolling.

本発明の一側面による耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板は、上述の合金組成を満たすだけではなく、表面に連続的または不連続的に１〜１００ｎｍの厚さのＳｉ非晶質酸化層が形成されている必要である。   The cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability according to one aspect of the present invention not only satisfies the above-described alloy composition but also has a thickness of 1 to 100 nm continuously or discontinuously on the surface. An amorphous oxide layer must be formed.

Ｓｉ非晶質酸化層は、Ｓｉが酸素と結合して生成される酸化物であって、鋼材に添加されたＳｉが焼鈍中に表層に濃化し、炉内に存在する酸素と結合して生成される非晶質構造を有する酸化物からなる層を意味する。Ｓｉ非晶質酸化層は腐食に強い化合物であって、耐食性の向上効果及び（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の形成を抑制するという効果がある。   The Si amorphous oxide layer is an oxide produced by combining Si with oxygen. Si added to the steel material is concentrated on the surface layer during annealing, and is formed by combining with oxygen present in the furnace. It means a layer made of an oxide having an amorphous structure. The Si amorphous oxide layer is a compound resistant to corrosion, and has an effect of improving the corrosion resistance and suppressing the formation of the (Fe, Mn, Cr) oxide layer.

その厚さが１ｎｍ未満の場合には、熱間成形後に十分なＳｉ非晶質酸化層が形成され難く、耐食性の向上効果が不十分となり、むしろ（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の形成を助長して十分な耐食性及び良好なスポット溶接性を確保し難い。   When the thickness is less than 1 nm, it is difficult to form a sufficient Si amorphous oxide layer after hot forming, and the effect of improving the corrosion resistance is insufficient. Rather, (Fe, Mn, Cr) oxide layer is formed. It is difficult to ensure sufficient corrosion resistance and good spot weldability.

一方、その厚さが１００ｎｍを超える場合には、熱間成形後に十分な耐食性は確保できるが、スポット溶接性を確保し難い。したがって、その厚さの上限は１００ｎｍであることが好ましく、より好ましい上限は７０ｎｍであり、さらに好ましい上限は５０ｎｍである。   On the other hand, when the thickness exceeds 100 nm, sufficient corrosion resistance can be ensured after hot forming, but it is difficult to ensure spot weldability. Therefore, the upper limit of the thickness is preferably 100 nm, the more preferable upper limit is 70 nm, and the more preferable upper limit is 50 nm.

このとき、上記Ｓｉ非晶質酸化層上に（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層が２．５μｍ以下の厚さで形成されることができる。   At this time, an (Fe, Mn, Cr) oxide layer may be formed on the Si amorphous oxide layer with a thickness of 2.5 μm or less.

（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さが２．５μｍを超える場合には、スポット溶接性の確保のために上記酸化物層を除去するためのショットブラスト工程が必要となり、耐食性を確保し難いという問題がある。   When the thickness of the (Fe, Mn, Cr) oxide layer exceeds 2.5 μm, a shot blasting process for removing the oxide layer is necessary to ensure spot weldability, ensuring corrosion resistance. There is a problem that it is difficult.

また、上記本発明による冷延鋼板の微細組織は、フェライト及びセメンタイトを含むことができる。特に、その面積分率を限定する必要はないが、例えば、５０面積％以上であることができる。   Moreover, the microstructure of the cold-rolled steel sheet according to the present invention can include ferrite and cementite. In particular, it is not necessary to limit the area fraction, but it can be, for example, 50 area% or more.

これは、熱間成形部材を製造するためにブランクを作る際に、上記冷延鋼板の強度が高すぎると、金型の摩耗が発生しやすくなるためである。しかし、このような現象を考慮しない場合には、ベイナイト、マルテンサイトなどを含むことができ、これを排除しない。   This is because, when making a blank for producing a hot-formed member, if the cold-rolled steel sheet is too strong, the mold is likely to be worn. However, when such a phenomenon is not considered, bainite, martensite, and the like can be included, and this is not excluded.

以下、本発明の他の一側面である熱間成形用冷延鋼板の製造方法について詳細に説明する。   Hereinafter, the manufacturing method of the cold-rolled steel sheet for hot forming which is the other side surface of this invention is demonstrated in detail.

本発明の他の一側面である熱間成形用冷延鋼板の製造方法は、上述の合金組成を満たすスラブを１０００〜１３００℃の温度で加熱する段階と、上記加熱されたスラブをＡｒ３〜１０００℃の仕上げ圧延温度で熱間圧延して熱延鋼板を得る段階と、上記熱延鋼板をＭｓ超７５０℃以下の温度範囲で巻き取る段階と、上記巻き取られた熱延鋼板を冷間圧延して冷延鋼板を得る段階と、上記冷延鋼板を、下記式（２）及び式（３）を満たすように連続焼鈍する段階と、を含む。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
式（２）：１≦ｅｘｐ［０．０７＊ＤＰ（Ｉ）＋（０．６＊Ｃｒ＋３＊Ｓｉ）］≦１００
式（３）：５０＊ｅｘｐ［０．０５＊ＤＰ（Ｉ）−（１．２＊Ｃｒ＋６＊Ｓｉ）］≦２．５
（上記式（１）から式（３）において各元素記号は、各元素の含量を重量％で測定した値であり、式（２）及び式（３）においてＤＰ（Ｉ）は、上記連続焼鈍段階の露点温度（℃）である。） The manufacturing method of the cold-rolled steel sheet for hot forming which is another aspect of the present invention includes a step of heating a slab satisfying the above-described alloy composition at a temperature of 1000 to 1300 ° C., and the heated slab as Ar 3 to 1000. A step of hot rolling at a finish rolling temperature of ℃ to obtain a hot-rolled steel plate, a step of winding the hot-rolled steel plate in a temperature range of over 750 ° C. below Ms, and cold rolling the wound hot-rolled steel plate And obtaining the cold-rolled steel sheet, and continuously annealing the cold-rolled steel sheet to satisfy the following formulas (2) and (3).
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
Formula (2): 1 ≦ exp [0.07 * DP (I) + (0.6 * Cr + 3 * Si)] ≦ 100
Formula (3): 50 * exp [0.05 * DP (I) − (1.2 * Cr + 6 * Si)] ≦ 2.5
(In the above formulas (1) to (3), each element symbol is a value obtained by measuring the content of each element in% by weight. In formulas (2) and (3), DP (I) is the above-mentioned continuous annealing. (Dew point temperature of the stage (℃))

（スラブ加熱段階）
上述の合金組成を満たすスラブを１０００〜１３００℃の温度で加熱する。 (Slab heating stage)
A slab satisfying the above alloy composition is heated at a temperature of 1000 to 1300 ° C.

上記加熱温度が１０００℃未満の場合には、スラブの組織を均質化し難く、１３００℃を超えると、酸化物が過剰に形成され、製造コストが上昇することがある。   When the heating temperature is less than 1000 ° C., it is difficult to homogenize the structure of the slab, and when it exceeds 1300 ° C., an oxide is excessively formed and the manufacturing cost may increase.

（熱間圧延段階）
上記加熱されたスラブをＡｒ３〜１０００℃の仕上げ圧延温度で熱間圧延して熱延鋼板を得る。 (Hot rolling stage)
The heated slab is hot-rolled at a finish rolling temperature of Ar 3 to 1000 ° C. to obtain a hot-rolled steel sheet.

仕上げ圧延温度がＡｒ３温度未満の場合には、二相域圧延になりやすくて表層に混粒組織が発生し、熱延鋼板の形状制御が困難になる。仕上げ圧延温度が１０００℃を超えると、熱延鋼板の結晶粒が粗大化しやすくなる。   When the finish rolling temperature is lower than the Ar3 temperature, two-phase rolling is likely to occur, and a mixed grain structure is generated in the surface layer, making it difficult to control the shape of the hot-rolled steel sheet. When the finish rolling temperature exceeds 1000 ° C., the crystal grains of the hot-rolled steel sheet are likely to be coarsened.

（巻取段階）
上記熱延鋼板をＭｓ超７５０℃以下の温度範囲で巻き取る。 (Winding stage)
The hot rolled steel sheet is wound up in a temperature range of more than Ms and 750 ° C. or less.

巻取温度がＭｓ（マルテンサイト変態開始温度）以下の場合には、熱延鋼板の強度が高くなりすぎて冷間圧延性を低下させる。巻取温度が７５０℃を超える場合には、酸化層の厚さが増加し、且つ表層の粒界酸化を引き起こして酸洗性に劣るようになるだけではなく、連続焼鈍炉での焼鈍時に表層の粒界の脱落を引き起こすという問題が発生することがある。   When the coiling temperature is equal to or lower than Ms (martensitic transformation start temperature), the strength of the hot-rolled steel sheet becomes too high and the cold rollability is lowered. When the coiling temperature exceeds 750 ° C., the thickness of the oxide layer increases, and not only does the surface layer undergo grain boundary oxidation, resulting in poor pickling properties, but also the surface layer during annealing in a continuous annealing furnace. The problem of causing dropout of grain boundaries may occur.

（冷間圧延段階）
上記巻き取られた熱延鋼板を冷間圧延して冷延鋼板を得る。これは、鋼板の厚さをより精密に制御するためであり、冷間圧延前に酸洗を行うことができる。 (Cold rolling stage)
The rolled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. This is for more precisely controlling the thickness of the steel sheet, and pickling can be performed before cold rolling.

このとき、上記冷間圧延の圧下率は、特に限定する必要はないが、所定の目標厚さを確保するために、３０〜８０％の圧下率で行うことができる。   At this time, the rolling reduction of the cold rolling is not particularly limited, but can be performed at a rolling reduction of 30 to 80% in order to secure a predetermined target thickness.

（連続焼鈍段階）
上記冷延鋼板を、下記式（２）及び式（３）を満たすように連続焼鈍する。下記式（２）及び式（３）において各元素記号は、各元素の含量を重量％で測定した値であり、ＤＰ（Ｉ）は、上記連続焼鈍段階の露点温度（℃）である。 (Continuous annealing stage)
The cold-rolled steel sheet is continuously annealed so as to satisfy the following formulas (2) and (3). In the following formulas (2) and (3), each element symbol is a value obtained by measuring the content of each element in% by weight, and DP (I) is a dew point temperature (° C.) in the continuous annealing stage.

式（２）：１≦ｅｘｐ［０．０７＊ＤＰ（Ｉ）＋（０．６＊Ｃｒ＋３＊Ｓｉ）］≦１００
式（２）は、Ｓｉ含量、Ｃｒ含量、及び連続焼鈍段階の露点温度（ＤＰ（Ｉ））の相関関係を考慮して、冷延鋼板のＳｉ非晶質酸化層の厚さを制御するためのものである。式（２）の値による冷延鋼板のＳｉ非晶質酸化層の厚さの変化を示したグラフである図２の（ａ）から確認できるように、式（２）の値によりＳｉ非晶質酸化層の厚さを制御することができる。 Formula (2): 1 ≦ exp [0.07 * DP (I) + (0.6 * Cr + 3 * Si)] ≦ 100
Equation (2) is for controlling the thickness of the Si amorphous oxide layer of the cold-rolled steel sheet in consideration of the correlation between the Si content, the Cr content, and the dew point temperature (DP (I)) of the continuous annealing stage. belongs to. As can be confirmed from FIG. 2 (a), which is a graph showing the change in the thickness of the Si amorphous oxide layer of the cold-rolled steel sheet according to the value of the formula (2), the value of the formula (2) allows Si amorphous The thickness of the quality oxide layer can be controlled.

上記式（２）の値が１未満の場合には、表面に十分な厚さのＳｉ非晶質酸化層を確保することができず、（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の形成を抑制し難く、別のめっき工程やショットブラスト工程がないと、優れたスポット溶接性及び耐食性を確保することができないという問題がある。   When the value of the above formula (2) is less than 1, a sufficiently thick Si amorphous oxide layer cannot be secured on the surface, and the formation of the (Fe, Mn, Cr) oxide layer is suppressed. However, if there is no separate plating process or shot blasting process, there is a problem that excellent spot weldability and corrosion resistance cannot be ensured.

上記式（２）の値が１００を超える場合には、Ｓｉ非晶質酸化層が厚くなりすぎてスポット溶接性を良好に確保し難いという問題がある。   When the value of the above formula (2) exceeds 100, there is a problem that the Si amorphous oxide layer becomes too thick and it is difficult to ensure good spot weldability.

式（３）：５０＊ｅｘｐ［０．０５＊ＤＰ（Ｉ）−（１．２＊Ｃｒ＋６＊Ｓｉ）］≦２．５
式（３）は、Ｓｉ含量、Ｃｒ含量、及び連続焼鈍段階の露点温度（ＤＰ（Ｉ））の相関関係を考慮して、冷延鋼板の（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さを制御するためのものである。式（３）の値による冷延鋼板のＳｉ非晶質酸化層の厚さの変化を示したグラフである図２の（ｂ）から確認できるように、式（３）の値により（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さを制御することができる。 Formula (3): 50 * exp [0.05 * DP (I) − (1.2 * Cr + 6 * Si)] ≦ 2.5
Equation (3) shows the thickness of the (Fe, Mn, Cr) oxide layer of the cold-rolled steel sheet in consideration of the correlation between the Si content, the Cr content, and the dew point temperature (DP (I)) of the continuous annealing stage. Is for controlling. As can be confirmed from FIG. 2 (b), which is a graph showing the change in the thickness of the Si amorphous oxide layer of the cold-rolled steel sheet according to the value of the formula (3), the value of the formula (3) (Fe, The thickness of the (Mn, Cr) oxide layer can be controlled.

式（３）の値が２．５を超える場合には、（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層が厚くなって鋼板表面の外観が悪くなるだけではなく、熱間成形後にもスポット溶接性を劣化させるという問題がある。   When the value of formula (3) exceeds 2.5, not only the (Fe, Mn, Cr) oxide layer becomes thick and the appearance of the steel sheet surface deteriorates, but also the spot weldability after hot forming. There is a problem of deteriorating.

このとき、上記連続焼鈍は７００〜９００℃の温度範囲で行うことができる。焼鈍温度が７００℃未満では、冷間圧延によって生成された圧延組織の回復及び再結晶が起こり難く、９００℃を超える場合には、焼鈍設備を劣化させるだけでなく、鋼板の表層に酸化物が過剰に形成されて、熱間成形後のスポット溶接性を大きく阻害する。   At this time, the continuous annealing can be performed in a temperature range of 700 to 900 ° C. When the annealing temperature is less than 700 ° C., recovery and recrystallization of the rolled structure generated by cold rolling hardly occur. It is formed excessively and greatly impairs the spot weldability after hot forming.

また、焼鈍時間は１〜１０００秒であることができる。本発明では、連続焼鈍を行うため、焼鈍時間を大幅に制御することは困難である。したがって、露点温度を制御しており、焼鈍時間が１秒未満の場合には、焼鈍効果を得難く、焼鈍時間が１０００秒を超える場合には、生産性が低下することがある。   Also, the annealing time can be 1-1000 seconds. In the present invention, since continuous annealing is performed, it is difficult to significantly control the annealing time. Therefore, when the dew point temperature is controlled and the annealing time is less than 1 second, it is difficult to obtain the annealing effect, and when the annealing time exceeds 1000 seconds, the productivity may decrease.

以下、本発明のさらに他の一側面である耐食性及びスポット溶接性に優れた熱間成形部材の製造方法について詳細に説明する。   Hereinafter, a method for producing a hot-formed member excellent in corrosion resistance and spot weldability, which is still another aspect of the present invention, will be described in detail.

本発明のさらに他の一側面である耐食性及びスポット溶接性に優れた熱間成形部材の製造方法は、上述の本発明による冷延鋼板の製造方法によって製造された冷延鋼板を、下記式（４）及び式（５）を満たす条件で１〜１０００℃／秒の昇温速度でＡｃ３〜Ａｃ３＋１５０℃の温度範囲まで加熱した後、１〜１０００秒間保持する熱処理段階と、上記加熱された冷延鋼板を熱間成形した後、１０〜１０００℃／秒の冷却速度で冷却する段階と、を含む。   The method for producing a hot-formed member excellent in corrosion resistance and spot weldability, which is still another aspect of the present invention, is obtained by replacing the cold-rolled steel sheet produced by the above-described method for producing a cold-rolled steel sheet with the following formula ( 4) After heating to a temperature range of Ac3 to Ac3 + 150 ° C. at a temperature rising rate of 1 to 1000 ° C./second under the conditions satisfying the formula (5), a heat treatment stage for holding for 1 to 1000 seconds, And hot-forming the steel sheet, followed by cooling at a cooling rate of 10 to 1000 ° C./second.

（熱処理段階）
上述の本発明による冷延鋼板の製造方法により製造された冷延鋼板を、下記式（４）及び式（５）を満たす条件で１〜１０００℃／秒の昇温速度でＡｃ３〜Ａｃ３＋１５０℃の温度範囲まで加熱した後、１〜１０００秒間維持して熱処理する。 (Heat treatment stage)
The cold-rolled steel sheet manufactured by the above-described method for manufacturing a cold-rolled steel sheet according to the present invention has a temperature increase rate of 1 to 1000 ° C./second under conditions that satisfy the following formulas (4) and (5). After heating to the temperature range, heat treatment is performed for 1 to 1000 seconds.

昇温速度が１℃／秒未満の場合には、生産性を十分に確保し難いだけではなく、部材の表面の酸化が過剰に促進され、十分なスポット溶接性を確保し難い。一方、昇温速度が１０００℃／秒を超える場合には、多くのコストがかかる設備が必要となる。   When the rate of temperature increase is less than 1 ° C./second, not only is it difficult to ensure sufficient productivity, but oxidation of the surface of the member is excessively promoted, and it is difficult to ensure sufficient spot weldability. On the other hand, when the rate of temperature rise exceeds 1000 ° C./second, equipment that requires a lot of cost is required.

加熱温度がＡｃ３未満であるか、または保持時間が１秒未満の場合には、オーステナイトに完全に変態しないフェライトが残存するだけではなく、ブランクが加熱炉から金型に移送される途中でフェライトが追加的に生成され得るため、所定の強度を確保し難いという問題がある。一方、加熱温度がＡｃ３＋１５０℃を超えるか、または保持時間が１０００秒を超える場合には、部材の表面に酸化物が過剰に生成されることにより、スポット溶接性を確保し難いという問題がある。   When the heating temperature is less than Ac3 or the holding time is less than 1 second, not only ferrite that does not completely transform into austenite remains, but also ferrite is transferred while the blank is being transferred from the heating furnace to the mold. Since it can be additionally generated, there is a problem that it is difficult to ensure a predetermined strength. On the other hand, when the heating temperature exceeds Ac3 + 150 ° C. or the holding time exceeds 1000 seconds, there is a problem that it is difficult to ensure spot weldability due to excessive generation of oxide on the surface of the member.

下記式（４）及び式（５）において各元素記号は、各元素の含量を重量％で測定した値であり、ＤＰ（ＩＩ）は、上記熱処理段階の露点温度（℃）である。   In the following formulas (4) and (5), each element symbol is a value obtained by measuring the content of each element in% by weight, and DP (II) is the dew point temperature (° C.) of the heat treatment stage.

式（４）：２≦式（２）＊ｅｘｐ［０．０７＊ＤＰ（ＩＩ）＋（０．６＊Ｃｒ＋１．５＊Ｓｉ）］≦２０００
式（４）は、Ｓｉ含量、Ｃｒ含量、連続焼鈍段階の露点温度（ＤＰ（Ｉ））、及び熱処理段階の露点温度（ＤＰ（ＩＩ））の相関関係を考慮して、熱間成形部材のＳｉ非晶質酸化層の厚さを制御するためのものである。式（４）の値による熱間成形部材のＳｉ非晶質酸化層の厚さの変化を示したグラフである図３の（ａ）から確認できるように、式（４）の値により熱間成形部材のＳｉ非晶質酸化層の厚さを制御することができる。 Formula (4): 2 ≦ Formula (2) * exp [0.07 * DP (II) + (0.6 * Cr + 1.5 * Si)] ≦ 2000
Equation (4) is obtained by considering the correlation among the Si content, the Cr content, the dew point temperature at the continuous annealing stage (DP (I)), and the dew point temperature at the heat treatment stage (DP (II)). This is for controlling the thickness of the Si amorphous oxide layer. As can be confirmed from FIG. 3A, which is a graph showing the change in the thickness of the Si amorphous oxide layer of the hot-formed member according to the value of the formula (4), the value of the formula (4) The thickness of the Si amorphous oxide layer of the molded member can be controlled.

上記式（４）の値が２未満の場合には、表面に十分な厚さのＳｉ非晶質酸化層を確保することができず、良好な耐食性を確保することができないという問題がある。したがって、式（４）の値の好ましい下限は２であり、より好ましくは３、さらに好ましくは４である。   When the value of the above formula (4) is less than 2, there is a problem that a Si amorphous oxide layer having a sufficient thickness cannot be secured on the surface, and good corrosion resistance cannot be secured. Therefore, the preferable lower limit of the value of the formula (4) is 2, more preferably 3, and still more preferably 4.

一方、上記式（４）の値が２０００を超える場合には、Ｓｉ非晶質酸化層が厚くなりすぎてスポット溶接性を良好に確保し難いという問題がある。   On the other hand, when the value of the above formula (4) exceeds 2000, there is a problem that the Si amorphous oxide layer becomes too thick and it is difficult to ensure good spot weldability.

式（５）：式（３）＋５０＊ｅｘｐ［０．０５＊ＤＰ（ＩＩ）−（０．４＊Ｃｒ＋２＊Ｓｉ）］≦３
式（５）は、Ｓｉ含量、Ｃｒ含量、連続焼鈍段階の露点温度（ＤＰ（Ｉ））、及び熱処理段階の露点温度（ＤＰ（ＩＩ））の相関関係を考慮して、熱間成形部材の（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さを制御するためのものである。式（５）の値による熱間成形部材の（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さの変化を示したグラフである図３の（ｂ）から確認できるように、式（５）の値により熱間成形部材の（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さを制御することができる。 Formula (5): Formula (3) + 50 * exp [0.05 * DP (II) − (0.4 * Cr + 2 * Si)] ≦ 3
Equation (5) is obtained by considering the correlation among the Si content, the Cr content, the dew point temperature at the continuous annealing stage (DP (I)), and the dew point temperature at the heat treatment stage (DP (II)). This is for controlling the thickness of the (Fe, Mn, Cr) oxide layer. As can be confirmed from FIG. 3 (b), which is a graph showing a change in the thickness of the (Fe, Mn, Cr) oxide layer of the hot-formed member according to the value of the equation (5), the equation (5) The thickness of the (Fe, Mn, Cr) oxide layer of the hot-formed member can be controlled by the value.

式（５）の値が３を超える場合は、（Ｆｅ、Ｍｎ、Ｃｒ）酸化物の厚さが厚くて鋼板表面の外観が悪くなるだけではなく、スポット溶接性に劣るようになるという問題がある。   When the value of formula (5) exceeds 3, not only the thickness of the (Fe, Mn, Cr) oxide is thick and the appearance of the steel sheet surface is deteriorated, but also the spot weldability is deteriorated. is there.

（熱間成形及び冷却段階）
上記加熱された冷延鋼板を熱間成形した後、１０〜１０００℃／秒の冷却速度で冷却する。 (Hot forming and cooling stage)
After the hot cold-rolled steel sheet is hot-formed, it is cooled at a cooling rate of 10 to 1000 ° C./second.

上記冷却速度が１０℃／ｓ未満の場合には、不要なフェライトが形成されて、１０００ＭＰａ以上の引張強度を確保し難い。一方、冷却速度を１０００℃／ｓ超に制御するためには、高価で特別な冷却設備が必要となる。   When the cooling rate is less than 10 ° C./s, unnecessary ferrite is formed, and it is difficult to secure a tensile strength of 1000 MPa or more. On the other hand, in order to control the cooling rate above 1000 ° C./s, expensive and special cooling equipment is required.

このとき、上記冷却する段階の冷却停止温度は、Ｍｆ（マルテンサイト変態終了温度）以下であることができる。これは、Ｍｆを超える温度で冷却を停止した後、常温まで再び冷却する場合、熱間成形部材の形状凍結性を確保し難いためである。   At this time, the cooling stop temperature in the cooling stage may be equal to or lower than Mf (martensitic transformation end temperature). This is because when the cooling is stopped at a temperature exceeding Mf and then cooled to room temperature again, it is difficult to ensure the shape freezing property of the hot-formed member.

但し、熱間成形部材においてより優れた伸びと衝撃特性を確保するために、Ｍｆ（マルテンサイト変態終了温度）とＭｓ（マルテンサイト変態開始温度）の間で冷却を停止した後、Ａｃ１以下の温度で再加熱してマルテンサイトを焼戻させ、残留オーステナイトを安定化させることもできる。   However, in order to ensure better elongation and impact characteristics in the hot-formed member, after cooling is stopped between Mf (martensitic transformation end temperature) and Ms (martensitic transformation start temperature), the temperature is less than Ac1. Can be reheated to temper the martensite and stabilize the retained austenite.

以下、本発明のさらに他の一側面である耐食性及びスポット溶接性に優れた熱間成形部材について詳細に説明する。   Hereinafter, a hot-formed member excellent in corrosion resistance and spot weldability, which is still another aspect of the present invention, will be described in detail.

本発明のさらに他の一側面である耐食性及びスポット溶接性に優れた熱間成形部材は、上述の合金組成を満たし、表面に連続的または不連続的に２ｎｍ〜２０００ｎｍの厚さのＳｉ非晶質酸化層が形成されている。   A hot-formed member excellent in corrosion resistance and spot weldability, which is still another aspect of the present invention, satisfies the above-described alloy composition and has a Si amorphous thickness of 2 nm to 2000 nm continuously or discontinuously on the surface. A quality oxide layer is formed.

Ｓｉ非晶質酸化層の厚さが２ｎｍ未満の場合には、十分な耐食性を確保し難い。したがって、その厚さの下限は２ｎｍであることが好ましく、より好ましくは３ｎｍ、さらに好ましくは３．５ｎｍである。   When the thickness of the Si amorphous oxide layer is less than 2 nm, it is difficult to ensure sufficient corrosion resistance. Therefore, the lower limit of the thickness is preferably 2 nm, more preferably 3 nm, and even more preferably 3.5 nm.

一方、その厚さが２０００ｎｍを超える場合には、十分な耐食性は確保できるが、良好なスポット溶接性を確保することは困難である。したがって、その厚さの上限は２０００ｎｍであることが好ましく、より好ましくは１０００ｎｍであり、さらに好ましくは５００ｎｍである。   On the other hand, when the thickness exceeds 2000 nm, sufficient corrosion resistance can be ensured, but it is difficult to ensure good spot weldability. Therefore, the upper limit of the thickness is preferably 2000 nm, more preferably 1000 nm, and even more preferably 500 nm.

このとき、上記Ｓｉ非晶質酸化層上に（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層が３μｍ以下の厚さに形成されていることができる。   At this time, an (Fe, Mn, Cr) oxide layer may be formed to a thickness of 3 μm or less on the Si amorphous oxide layer.

（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さが３μｍを超える場合には、鋼板表面の外観が悪くなるだけではなく、スポット溶接性の確保のために上記酸化物層を除去するためのショットブラスト工程が必要となり、耐食性を確保し難いという問題点がある。   When the thickness of the (Fe, Mn, Cr) oxide layer exceeds 3 μm, not only the appearance of the steel sheet surface is deteriorated, but also a shot for removing the oxide layer to ensure spot weldability. A blasting process is required, and it is difficult to ensure corrosion resistance.

また、上記熱間成形部材は、高強度を確保するために、マルテンサイトまたはベイナイトを主相にすることができる。ここで主相とは、微細組織をなす複数の相の中で最も大きい面積分率を有する相を意味する。その面積分率を特に限定する必要はないが、例えば、５０面積％以上であることができる。   Moreover, in order to ensure the high intensity | strength, the said hot forming member can make a martensite or a bainite the main phase. Here, the main phase means a phase having the largest area fraction among a plurality of phases forming a fine structure. The area fraction need not be particularly limited, but can be, for example, 50 area% or more.

一方、上記熱間成形部材は、１０００ＭＰａ以上の引張強度を有することができる。１０００ＭＰａ以上の高強度を確保することにより、耐衝突性が求められる自動車構造部材または補強材などに好適に適用されることができる。   On the other hand, the hot-formed member can have a tensile strength of 1000 MPa or more. By securing a high strength of 1000 MPa or more, it can be suitably applied to automobile structural members or reinforcing materials that require crash resistance.

また、上記熱間成形部材は、スポット溶接電流範囲が１．０ｋＡ以上であることができる。通常、顧客社では１．０ｋＡ以上のスポット溶接電流範囲を必要としているため、スポット溶接電流範囲が１．０ｋＡの場合、スポット溶接性に劣る。   The hot-formed member may have a spot welding current range of 1.0 kA or more. Usually, since the customer company requires a spot welding current range of 1.0 kA or more, when the spot welding current range is 1.0 kA, the spot weldability is poor.

以下、実施例を挙げて本発明をより具体的に説明する。但し、下記の実施例は、本発明を例示してより詳細に説明するためのものであり、本発明の権利範囲を限定するためのものではないという点に留意する必要がある。本発明の権利範囲は、特許請求の範囲に記載された事項とそれから合理的に類推される事項によって決定されるものである。   Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are for illustrating the present invention in more detail and are not intended to limit the scope of rights of the present invention. The scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

（実施例１）
下記表１に示した成分組成を有する厚さ４０ｍｍのスラブを真空溶解し、１２００℃の加熱炉で１時間加熱した後、９００℃の仕上げ圧延温度で熱間圧延して最終厚さ３ｍｍの熱延鋼板を製造した。その後、上記熱延鋼板を６００℃の温度で巻き取ってから、上記熱延鋼板を酸洗した後、５０％の冷間圧下率で冷間圧延を行った。 (Example 1)
A 40 mm thick slab having the composition shown in Table 1 below was melted in vacuum, heated in a heating furnace at 1200 ° C. for 1 hour, and then hot-rolled at a finish rolling temperature of 900 ° C. to give a final thickness of 3 mm. A rolled steel sheet was produced. Then, after the said hot-rolled steel plate was wound up at the temperature of 600 degreeC, after pickling the said hot-rolled steel plate, it cold-rolled with the cold reduction rate of 50%.

上述のように製造された冷延鋼板を用いて、表２に示すように７８０℃の焼鈍温度で露点温度の条件（ＤＰ（Ｉ））を変更して連続焼鈍を行った。連続焼鈍後、冷延鋼板の表面のＳｉ非晶質酸化層の厚さ及び（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さを測定して下記表２に記載した。また、下記式（１）から式（３）の値を計算して下記表３に記載した。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
式（２）：１≦ｅｘｐ［０．０７＊ＤＰ（Ｉ）＋（０．６＊Ｃｒ＋３＊Ｓｉ）］≦１００
式（３）：５０＊ｅｘｐ［０．０５＊ＤＰ（Ｉ）−（１．２＊Ｃｒ＋６＊Ｓｉ）］≦２．５
（上記式（１）から式（３）において各元素記号は、各元素の含量を重量％で測定した値であり、式（２）及び式（３）においてＤＰ（Ｉ）は、上記連続焼鈍段階の露点温度（℃）である。） Using the cold-rolled steel sheet produced as described above, continuous annealing was performed by changing the dew point temperature condition (DP (I)) at an annealing temperature of 780 ° C. as shown in Table 2. After continuous annealing, the thickness of the Si amorphous oxide layer on the surface of the cold rolled steel sheet and the thickness of the (Fe, Mn, Cr) oxide layer were measured and listed in Table 2 below. Further, the values of the following formulas (1) to (3) were calculated and listed in Table 3 below.
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
Formula (2): 1 ≦ exp [0.07 * DP (I) + (0.6 * Cr + 3 * Si)] ≦ 100
Formula (3): 50 * exp [0.05 * DP (I) − (1.2 * Cr + 6 * Si)] ≦ 2.5
(In the above formulas (1) to (3), each element symbol is a value obtained by measuring the content of each element in% by weight. In formulas (2) and (3), DP (I) is the above-mentioned continuous annealing. (Dew point temperature of the stage (℃))

Ｓｉ非晶質酸化層の厚さ及び（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さは、透過電子顕微鏡（ＴＥＭ）及びＥＰＭＡ機器を用いて３箇所を測定した後、平均した結果値を示した。図４と図５は、発明例Ａ２の代表的な表層構造及び成分分布を示す。発明例Ａ１〜Ａ４は、式（２）と式（３）を満たすのに対し、比較例Ａ５〜Ａ６及びＢ１〜Ｂ５は、式（２）または式（３）を満たしていない。   The thickness of the Si amorphous oxide layer and the thickness of the (Fe, Mn, Cr) oxide layer are average values after measuring three locations using a transmission electron microscope (TEM) and EPMA equipment. It was. 4 and 5 show a typical surface layer structure and component distribution of Invention Example A2. Inventive Examples A1 to A4 satisfy Expressions (2) and (3), whereas Comparative Examples A5 to A6 and B1 to B5 do not satisfy Expression (2) or Expression (3).

上述のように製造された冷延鋼板を用いて熱間成形を行った。このときの熱処理条件としては、下記表３に記載された露点温度（ＤＰ（ＩＩ））に制御し、９００℃の温度に予め加熱された加熱炉に上記冷延鋼板を装入した後、６分間保持した。次に、１２秒間空冷した後、金型で熱間成形してから１０℃／秒以上の冷却速度で室温まで急冷して熱間成形部材を得た。上記熱間成形部材の引張強度、表面等級、Ｓｉ非晶質酸化層の厚さ、（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さ、耐食性、及びスポット溶接性を測定または評価して下記表３に記載した。また、下記式（４）及び式（５）の値を計算して下記表３に記載した。
式（４）：２≦式（２）＊ｅｘｐ［０．０７＊ＤＰ（ＩＩ）＋（０．６＊Ｃｒ＋１．５＊Ｓｉ）］≦２０００
式（５）：式（３）＋５０＊ｅｘｐ［０．０５＊ＤＰ（ＩＩ）−（０．４＊Ｃｒ＋２＊Ｓｉ）］≦３
（上記式（４）及び式（５）において各元素記号は、各元素の含量を重量％で測定した値であり、ＤＰ（ＩＩ）は、上記熱処理段階の露点温度（℃）である。） Hot forming was performed using the cold-rolled steel sheet produced as described above. As heat treatment conditions at this time, the dew point temperature (DP (II)) described in Table 3 below was controlled, and after the cold-rolled steel sheet was charged into a heating furnace preheated to a temperature of 900 ° C., 6 Hold for a minute. Next, after air cooling for 12 seconds, hot forming was performed with a mold, and then rapidly cooled to room temperature at a cooling rate of 10 ° C./second or more to obtain a hot formed member. The following table was measured or evaluated for the tensile strength, surface grade, Si amorphous oxide layer thickness, (Fe, Mn, Cr) oxide layer thickness, corrosion resistance, and spot weldability of the hot-formed member. 3. Moreover, the value of the following formula (4) and formula (5) was calculated and listed in Table 3 below.
Formula (4): 2 ≦ Formula (2) * exp [0.07 * DP (II) + (0.6 * Cr + 1.5 * Si)] ≦ 2000
Formula (5): Formula (3) + 50 * exp [0.05 * DP (II) − (0.4 * Cr + 2 * Si)] ≦ 3
(In the above formulas (4) and (5), each element symbol is a value obtained by measuring the content of each element in% by weight, and DP (II) is the dew point temperature (° C.) of the heat treatment stage.)

引張強度は、上記熱間成形部材からＡＳＴＭ Ｅ８引張試験片を採取して測定し、Ｓｉ非晶質酸化層の厚さ及び（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さは、透過電子顕微鏡（ＴＥＭ）及びＥＰＭＡ機器を用いて３箇所を測定した後、平均した結果値を示した。熱間成形後、代表的な発明例Ａ２−１の表層構造及び成分分布は図６及び７に示した。   Tensile strength is measured by collecting ASTM E8 tensile test pieces from the hot-formed members, and the thickness of the Si amorphous oxide layer and the thickness of the (Fe, Mn, Cr) oxide layer are measured by a transmission electron microscope. After measuring three locations using (TEM) and EPMA equipment, the averaged result values are shown. After hot forming, the surface layer structure and component distribution of representative invention example A2-1 are shown in FIGS.

表面等級は、ショットブラストが行われていない表面を目視で評価した。つまり、部材の表面が厚い酸化層で構成されていて表面の色が濃い灰色に見える場合の面積率と、部材の表面が薄い酸化層で構成されていて表面の色が黄色または金色に見える場合の面積率を計算して下記のように評価した。
５等級：灰色表面の面積率が９０％超
４等級：灰色表面の面積率が７０％超９０％以下
３等級：灰色表面の面積率が３０％超７０％以下
２等級：灰色表面の面積率が１０％超３０％以下
１等級：灰色表面の面積率が１０％以下 The surface grade was evaluated by visual observation of a surface that was not shot blasted. In other words, the area ratio when the surface of the member is composed of a thick oxide layer and the surface color appears dark gray, and the surface ratio of the member is composed of a thin oxide layer and the surface color appears yellow or gold The area ratio was calculated and evaluated as follows.
Grade 5: Gray surface area ratio is more than 90% Grade 4: Gray surface area ratio is more than 70% and less than 90% Grade 3: Gray surface area ratio is more than 30% and less than 70% Grade 2: Gray surface area ratio Is more than 10% and 30% or less Grade 1: Gray surface area ratio is 10% or less

耐食性は、塩水噴霧（ｓａｌｔ ｓｐｒａｙ）を用いて、熱間成形部材に対するＣＣＴ（Ｃｙｃｌｉｃ ｃｏｒｒｏｓｉｏｎ ｔｅｓｔ）を６３回行った後、腐食の深さを２ｍｍの間隔で３個所測定して平均した。上記腐食の深さが１ｍｍを超えた場合を不良（×）、１ｍｍ以下の場合を良好（Ｏ）と判断した。   The corrosion resistance was averaged by measuring the depth of corrosion at 3 locations at intervals of 2 mm after 63 times of CCT (Cyclic Corrosion Test) on the hot-formed member using salt spray. The case where the depth of the corrosion exceeded 1 mm was judged as poor (x), and the case where it was 1 mm or less was judged as good (O).

スポット溶接性は、ＩＳＯ規格（１８２７８−２）を用いてスポット溶接電流範囲を求め、電流範囲が１．０ｋＡ以上の場合を良好、１．０ｋＡ未満の場合を不良とした。   For spot weldability, the spot welding current range was obtained using ISO standard (18278-2), and the case where the current range was 1.0 kA or higher was judged good, and the case where the current range was less than 1.0 kA was regarded as bad.

本発明の式（１）の値を満たす発明鋼Ａの場合、すべての熱間成形熱処理条件下で４等級未満の等級を示した。   Inventive steel A satisfying the value of formula (1) of the present invention showed a grade of less than 4 under all hot forming heat treatment conditions.

一方、Ｓｉ、Ｃｒ及び式（１）の値を満たしていない比較鋼Ｂの場合、すべての熱間成形熱処理条件下で表面等級が５等級を示し、表面特性に劣った。各元素の含量の範囲は本発明の範囲を満たすが、式（１）の値を満たしてない比較鋼Ｃも、表面等級が４等級を示し、表面特性に劣った。   On the other hand, in the case of the comparative steel B that does not satisfy the values of Si, Cr and the formula (1), the surface grade was 5 under all hot forming heat treatment conditions, and the surface properties were inferior. Although the range of the content of each element satisfies the range of the present invention, the comparative steel C not satisfying the value of the formula (1) also showed a surface grade of 4 and was inferior in surface characteristics.

また、発明例Ａ１〜Ａ４の場合は、耐食性及びスポット溶接性を両方とも確保できる熱間成形部材を製造することができた。   Moreover, in the case of invention example A1-A4, the hot forming member which can ensure both corrosion resistance and spot weldability was able to be manufactured.

一方、本発明の合金組成は満たすが、本発明による冷延鋼板の条件を満たしていない比較例Ａ５〜Ａ６は、耐食性は確保することができたが、スポット溶接性は確保することができなかった。   On the other hand, Comparative Examples A5 to A6 that satisfy the alloy composition of the present invention but do not satisfy the conditions of the cold-rolled steel sheet according to the present invention can ensure corrosion resistance, but cannot ensure spot weldability. It was.

また、Ａ１−２の場合、本発明による冷延鋼板の条件は満たしたが、式（４）の値が２未満を示し、熱間成形部材の耐食性に劣った。Ａ１−３の場合、本発明による冷延鋼板の条件は満たしたが、式（５）の値が３を超えたため、熱間成形部材のスポット溶接性に劣った。   Moreover, in the case of A1-2, although the conditions of the cold-rolled steel sheet by this invention were satisfy | filled, the value of Formula (4) showed less than 2, and it was inferior to the corrosion resistance of a hot forming member. In the case of A1-3, although the conditions of the cold-rolled steel sheet according to the present invention were satisfied, the value of the formula (5) exceeded 3, so that the spot weldability of the hot-formed member was inferior.

（実施例２）
本発明の範囲内で１０００ＭＰａ以上の引張強度、優れた耐食性、及びスポット溶接性を確保できることをより明確に確認するために追加実験を行った。 (Example 2)
In order to more clearly confirm that a tensile strength of 1000 MPa or more, excellent corrosion resistance, and spot weldability can be secured within the scope of the present invention, an additional experiment was conducted.

下記表４のような組成を有する厚さ４０ｍｍのスラブに対して、下記表５の連続焼鈍段階の露点温度、下記表６の熱処理段階の露点温度、及び残りの製造条件は上記実施例１と同一の条件とし、冷延鋼板と熱間成形部材を製造した。   For a slab having a thickness of 40 mm having the composition shown in Table 4 below, the dew point temperature in the continuous annealing stage in Table 5 below, the dew point temperature in the heat treatment stage in Table 6 below, and the remaining production conditions are the same as in Example 1 above. Under the same conditions, a cold-rolled steel sheet and a hot-formed member were manufactured.

上記冷延鋼板の表面のＳｉ非晶質酸化層の厚さ及び（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さを測定して下記表５に記載した。   The thickness of the Si amorphous oxide layer on the surface of the cold-rolled steel sheet and the thickness of the (Fe, Mn, Cr) oxide layer were measured and listed in Table 5 below.

上記熱間成形部材の引張強度、表面等級、Ｓｉ非晶質酸化層の厚さ、（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層の厚さ、耐食性、及びスポット溶接性を測定または評価して下記表６に記載した。   The following table was measured or evaluated for the tensile strength, surface grade, Si amorphous oxide layer thickness, (Fe, Mn, Cr) oxide layer thickness, corrosion resistance, and spot weldability of the hot-formed member. 6.

測定及び評価方法は実施例１と同一にした。   The measurement and evaluation method was the same as in Example 1.

上記表６から確認できるように、発明例Ｄ１−１からＫ１−１はいずれも、本発明の合金組成及び製造条件を満たし、優れた耐食性及びスポット溶接性を確保することができた。   As can be seen from Table 6 above, Invention Examples D1-1 to K1-1 all satisfied the alloy composition and production conditions of the present invention, and were able to ensure excellent corrosion resistance and spot weldability.

以上の実施例を参照して説明したが、当該技術分野における熟練した当業者は、添付の特許請求の範囲に記載された本発明の思想及び領域から逸脱しない範囲内で、本発明を多様に修正及び変更させることができることを理解することができる。   Although described with reference to the above-described embodiments, those skilled in the art can make various changes to the present invention without departing from the spirit and scope of the present invention described in the appended claims. It can be understood that modifications and changes can be made.

Claims (20)

重量％で、Ｃ：０．１〜０．４％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．０１〜４．０％、Ａｌ：０．００１〜０．４％、Ｐ：０．００１〜０．０５％、Ｓ：０．０００１〜０．０２％、Ｃｒ：０．５％以上３．０％未満、Ｎ：０．００１〜０．０２％、残りＦｅ及びその他の不可避不純物を含み、下記式（１）を満たし、
表面に連続的または不連続的に１ｎｍ〜１００ｎｍの厚さのＳｉ非晶質酸化層が形成されている、耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
（前記式（１）において各元素記号は、各元素の含量を重量％で測定した値である。） By weight, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.5% to less than 3.0%, N: 0.001 to 0.02%, remaining Fe and other inevitable Contains impurities, satisfies the following formula (1),
A cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability, wherein a Si amorphous oxide layer having a thickness of 1 nm to 100 nm is continuously or discontinuously formed on the surface.
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
(In the formula (1), each element symbol is a value obtained by measuring the content of each element in% by weight.) 前記Ｓｉ非晶質酸化層上に（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層が２．５μｍ以下の厚さに形成されている、請求項１に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板。   The hot forming excellent in corrosion resistance and spot weldability according to claim 1, wherein a (Fe, Mn, Cr) oxide layer is formed to a thickness of 2.5 μm or less on the Si amorphous oxide layer. Cold rolled steel sheet. 前記冷延鋼板は、重量％で、下記ａ）及びｂ）の中から選択された１以上をさらに含む、請求項１に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板。
ａ）Ｔｉ、Ｎｂ、Ｚｒ及びＶから選択された１種以上：０．００１〜０．４％
ｂ）Ｂ：０．０００１〜０．０１％ The cold-rolled steel sheet for hot forming according to claim 1, wherein the cold-rolled steel sheet further includes one or more selected from the following a) and b) by weight%.
a) One or more selected from Ti, Nb, Zr and V: 0.001 to 0.4%
b) B: 0.0001 to 0.01% 前記冷延鋼板は、重量％で、下記ｃ）からｅ）の中から選択された１以上をさらに含む、請求項１に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板。
ｃ）Ｍｏ及びＷから選択された１種以上：０．００１〜１．０％
ｄ）ＣｕとＮｉ含量の合計：０．００５〜２．０％
ｅ）Ｓｂ及びＳｎから選択された１種以上：０．００１〜１．０％ The cold-rolled steel sheet for hot forming according to claim 1, wherein the cold-rolled steel sheet further includes at least one selected from the following c) to e) by weight% and excellent in corrosion resistance and spot weldability.
c) One or more selected from Mo and W: 0.001 to 1.0%
d) Total Cu and Ni content: 0.005 to 2.0%
e) one or more selected from Sb and Sn: 0.001 to 1.0% 前記冷延鋼板の微細組織は、フェライト及びセメンタイトを含む、請求項１に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板。   The hot-rolled cold-rolled steel sheet having excellent corrosion resistance and spot weldability according to claim 1, wherein the microstructure of the cold-rolled steel sheet includes ferrite and cementite. 重量％で、Ｃ：０．１〜０．４％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．０１〜４．０％、Ａｌ：０．００１〜０．４％、Ｐ：０．００１〜０．０５％、Ｓ：０．０００１〜０．０２％、Ｃｒ：０．５％以上３．０％未満、Ｎ：０．００１〜０．０２％、残りＦｅ及びその他の不可避不純物を含み、下記式（１）を満たすスラブを１０００〜１３００℃の温度で加熱する段階と、
前記加熱されたスラブをＡｒ３〜１０００℃の仕上げ圧延温度で熱間圧延して熱延鋼板を得る段階と、
前記熱延鋼板をＭｓ超７５０℃以下の温度範囲で巻き取る段階と、
前記巻き取られた熱延鋼板を冷間圧延して冷延鋼板を得る段階と、
前記冷延鋼板を、下記式（２）及び式（３）を満たすように連続焼鈍する段階と、を含む、耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板の製造方法。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
式（２）：１≦ｅｘｐ［０．０７＊ＤＰ（Ｉ）＋（０．６＊Ｃｒ＋３＊Ｓｉ）］≦１００
式（３）：５０＊ｅｘｐ［０．０５＊ＤＰ（Ｉ）−（１．２＊Ｃｒ＋６＊Ｓｉ）］≦２．５
（前記式（１）から式（３）において各元素記号は、各元素の含量を重量％で測定した値であり、式（２）及び式（３）においてＤＰ（Ｉ）は、前記連続焼鈍段階の露点温度（℃）である。） By weight, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.5% to less than 3.0%, N: 0.001 to 0.02%, remaining Fe and other inevitable Heating a slab containing impurities and satisfying the following formula (1) at a temperature of 1000 to 1300 ° C .;
Hot-rolling the heated slab at a finishing rolling temperature of Ar 3 to 1000 ° C. to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet in a temperature range of more than Ms and not more than 750 ° C .;
Cold rolling the wound hot rolled steel sheet to obtain a cold rolled steel sheet;
The manufacturing method of the cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability including the step of continuously annealing the cold-rolled steel sheet to satisfy the following formulas (2) and (3).
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
Formula (2): 1 ≦ exp [0.07 * DP (I) + (0.6 * Cr + 3 * Si)] ≦ 100
Formula (3): 50 * exp [0.05 * DP (I) − (1.2 * Cr + 6 * Si)] ≦ 2.5
(In the above formulas (1) to (3), each element symbol is a value obtained by measuring the content of each element in% by weight. In the formulas (2) and (3), DP (I) is the above-mentioned continuous annealing. (Dew point temperature of the stage (℃)) 前記スラブは、重量％で、下記ａ）及びｂ）の中から選択された１以上をさらに含む、請求項６に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板の製造方法。
ａ）Ｔｉ、Ｎｂ、Ｚｒ及びＶから選択された１種以上：０．００１〜０．４％
ｂ）Ｂ：０．０００１〜０．０１％ The method for producing a cold-rolled steel sheet for hot forming with excellent corrosion resistance and spot weldability according to claim 6, wherein the slab further comprises one or more selected from the following a) and b) by weight%. .
a) One or more selected from Ti, Nb, Zr and V: 0.001 to 0.4%
b) B: 0.0001 to 0.01% 前記スラブは、重量％で、下記ｃ）からｅ）の中から選択された１以上をさらに含む、請求項６に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板の製造方法。
ｃ）Ｍｏ及びＷから選択された１種以上：０．００１〜１．０％
ｄ）ＣｕとＮｉ含量の合計：０．００５〜２．０％
ｅ）Ｓｂ及びＳｎから選択された１種以上：０．００１〜１．０％ The method for producing a cold-rolled steel sheet for hot forming excellent in corrosion resistance and spot weldability according to claim 6, wherein the slab further includes one or more selected from the following c) to e) by weight%. .
c) One or more selected from Mo and W: 0.001 to 1.0%
d) Total Cu and Ni content: 0.005 to 2.0%
e) one or more selected from Sb and Sn: 0.001 to 1.0% 前記冷間圧延は、３０〜８０％の圧下率で行われる、請求項６に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板の製造方法。   The said cold rolling is a manufacturing method of the cold-rolled steel plate for hot forming excellent in the corrosion resistance and spot weldability of Claim 6 performed by the reduction rate of 30 to 80%. 前記連続焼鈍は、７００〜９００℃の温度範囲で行われる、請求項６に記載の耐食性及びスポット溶接性に優れた熱間成形用冷延鋼板の製造方法。   The said continuous annealing is a manufacturing method of the cold-rolled steel sheet for hot forming excellent in the corrosion resistance and spot weldability of Claim 6 performed in the temperature range of 700-900 degreeC. 重量％で、Ｃ：０．１〜０．４％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．０１〜４．０％、Ａｌ：０．００１〜０．４％、Ｐ：０．００１〜０．０５％、Ｓ：０．０００１〜０．０２％、Ｃｒ：０．５％以上３．０％未満、Ｎ：０．００１〜０．０２％、残りＦｅ及びその他の不可避不純物を含み、下記式（１）を満たし、
表面に連続的または不連続的に２ｎｍ〜２０００ｎｍの厚さのＳｉ非晶質酸化層が形成されている、耐食性及びスポット溶接性に優れた熱間成形部材。
式（１）：１．４≦０．４＊Ｃｒ＋Ｓｉ≦３．２
（前記式（１）において各元素記号は、各元素の含量を重量％で測定した値である。） By weight, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.5% to less than 3.0%, N: 0.001 to 0.02%, remaining Fe and other inevitable Contains impurities, satisfies the following formula (1),
A hot-formed member excellent in corrosion resistance and spot weldability, wherein a Si amorphous oxide layer having a thickness of 2 nm to 2000 nm is continuously or discontinuously formed on the surface.
Formula (1): 1.4 ≦ 0.4 * Cr + Si ≦ 3.2
(In the formula (1), each element symbol is a value obtained by measuring the content of each element in% by weight.) 前記Ｓｉ非晶質酸化層上に（Ｆｅ、Ｍｎ、Ｃｒ）酸化物層が３μｍ以下の厚さに形成されている、請求項１１に記載の耐食性及びスポット溶接性に優れた熱間成形部材。   The hot-formed member excellent in corrosion resistance and spot weldability according to claim 11, wherein a (Fe, Mn, Cr) oxide layer is formed to a thickness of 3 µm or less on the Si amorphous oxide layer. 前記熱間成形部材は、重量％で、下記ａ）及びｂ）の中から選択された１以上をさらに含む、請求項１１に記載の耐食性及びスポット溶接性に優れた熱間成形部材。
ａ）Ｔｉ、Ｎｂ、Ｚｒ及びＶから選択された１種以上：０．００１〜０．４％
ｂ）Ｂ：０．０００１〜０．０１％ The hot-formed member excellent in corrosion resistance and spot weldability according to claim 11, wherein the hot-formed member further includes one or more selected from the following a) and b) by weight%.
a) One or more selected from Ti, Nb, Zr and V: 0.001 to 0.4%
b) B: 0.0001 to 0.01% 前記熱間成形部材は、重量％で、下記ｃ）からｅ）の中から選択された１以上をさらに含む、請求項１１に記載の耐食性及びスポット溶接性に優れた熱間成形部材。
ｃ）Ｍｏ及びＷから選択された１種以上：０．００１〜１．０％
ｄ）ＣｕとＮｉ含量の合計：０．００５〜２．０％
ｅ）Ｓｂ及びＳｎから選択された１種以上：０．００１〜１．０％ The hot-formed member excellent in corrosion resistance and spot weldability according to claim 11, wherein the hot-formed member further includes one or more selected from the following c) to e) by weight%.
c) One or more selected from Mo and W: 0.001 to 1.0%
d) Total Cu and Ni content: 0.005 to 2.0%
e) one or more selected from Sb and Sn: 0.001 to 1.0% 前記熱間成形部材は、マルテンサイトまたはベイナイトを主相とする、請求項１１に記載の耐食性及びスポット溶接性に優れた熱間成形部材。   The hot forming member according to claim 11, wherein the hot forming member has martensite or bainite as a main phase and is excellent in corrosion resistance and spot weldability. 前記熱間成形部材は、１０００ＭＰａ以上の引張強度を有する、請求項１１に記載の耐食性及びスポット溶接性に優れた熱間成形部材。   The hot-formed member excellent in corrosion resistance and spot weldability according to claim 11, wherein the hot-formed member has a tensile strength of 1000 MPa or more. 前記熱間成形部材は、スポット溶接電流範囲が１．０ｋＡ以上である、請求項１１に記載の耐食性及びスポット溶接性に優れた熱間成形部材。   The hot-formed member having excellent corrosion resistance and spot weldability according to claim 11, wherein the hot-formed member has a spot welding current range of 1.0 kA or more. 請求項６から１０のいずれか一項によって製造された冷延鋼板を、下記式（４）及び式（５）を満たす条件で１〜１０００℃／秒の昇温速度でＡｃ３〜Ａｃ３＋１５０℃の温度範囲まで加熱した後、１〜１０００秒間保持する熱処理段階と、
前記加熱された冷延鋼板を熱間成形した後、１０〜１０００℃／秒の冷却速度で冷却する段階と、を含む、耐食性及びスポット溶接性に優れた熱間成形部材の製造方法。
式（４）：２≦式（２）＊ｅｘｐ［０．０７＊ＤＰ（ＩＩ）＋（０．６＊Ｃｒ＋１．５＊Ｓｉ）］≦２０００
式（５）：式（３）＋５０＊ｅｘｐ［０．０５＊ＤＰ（ＩＩ）−（０．４＊Ｃｒ＋２＊Ｓｉ）］≦３
（前記式（４）及び式（５）において各元素記号は、各元素の含量を重量％で測定した値であり、ＤＰ（ＩＩ）は、前記熱処理段階の露点温度（℃）である。） The temperature of Ac3-Ac3 + 150 degreeC with the temperature increase rate of 1-1000 degree-C / sec on the conditions which satisfy | fill following formula (4) and Formula (5) the cold-rolled steel plate manufactured by any one of Claim 6 to 10. A heat treatment stage for 1 to 1000 seconds after heating to a range; and
A method for producing a hot-formed member excellent in corrosion resistance and spot weldability, comprising hot-forming the heated cold-rolled steel sheet and then cooling it at a cooling rate of 10 to 1000 ° C./second.
Formula (4): 2 ≦ Formula (2) * exp [0.07 * DP (II) + (0.6 * Cr + 1.5 * Si)] ≦ 2000
Formula (5): Formula (3) + 50 * exp [0.05 * DP (II) − (0.4 * Cr + 2 * Si)] ≦ 3
(In the above formulas (4) and (5), each element symbol is a value obtained by measuring the content of each element in% by weight, and DP (II) is the dew point temperature (° C.) of the heat treatment stage.) 前記冷却する段階の冷却停止温度は、Ｍｆ（マルテンサイト変態終了温度）以下である、請求項１８に記載の耐食性及びスポット溶接性に優れた熱間成形部材の製造方法。   The method for producing a hot-formed member excellent in corrosion resistance and spot weldability according to claim 18, wherein a cooling stop temperature at the cooling stage is Mf (martensitic transformation end temperature) or lower. 前記冷却する段階の冷却停止温度は、Ｍｆ（マルテンサイト変態終了温度）〜Ｍｓ（マルテンサイト変態開始温度）であり、
前記冷却後、Ａｃ１以下の温度で加熱して焼戻する段階をさらに含む、請求項１８に記載の耐食性及びスポット溶接性に優れた熱間成形部材の製造方法。 The cooling stop temperature in the cooling stage is Mf (martensitic transformation end temperature) to Ms (martensitic transformation start temperature),
The method for producing a hot-formed member excellent in corrosion resistance and spot weldability according to claim 18, further comprising a step of heating and tempering at a temperature of Ac1 or lower after the cooling.
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